1. Field of the Invention
The present invention in general relates to a thick film conductor paste, and in particular to a thick film conductor composition with superior wear resistance that is used to prevent the fogging of windows in an automobile.
2. Description of Related Art
An important application of patterned electrically-conductive layers is in the automobile manufacturing industry, and particularly in the manufacture of windows which can be defrosted and/or demisted by an electrically-conductive grid permanently attached to the window and capable of producing heat when powered by a voltage source. In order for the window to defrost quickly, the circuit must be capable of supplying large amounts of power from a low voltage power source, typically 12 volts. For such power sources the resistivity requirement of the conductive pattern is generally in the range of from about 2 to about 5 μΩ cm (5 mΩ/square at 10 μm after firing). This requirement is readily met by conductors containing noble metals, particularly silver which is the most commonly-used material for this application.
The substances used in the production of grids for defogging windows include primarily, thick-film silver conductors which are manufactured from a paste composed of fine particles of silver powder and glass frit dispersed in an organic medium. In a typical application, a paste containing 70 wt % of silver powder, 5 wt % of glass frit and 25 wt % of organic medium is screen-printed through a 180 standard mesh screen onto flat, un-formed rear window glass. The printed composition is dried at about 150° C. for at least 2 minutes, following which the entire element is fired in air at 650° C. for 2 to 5 minutes. After firing, the softened window glass is shaped by compression in a mold, then quenched by rapid cooling. The organic medium is removed by vaporization and pyrolysis in the firing cycle. A continuous electrically conductive path is formed by sintering the glass and silver, and having the glass act as a binder for the silver particles.
The prior art relating to such thick-film conductor pastes for automotive glass includes the thick-film paste composition for use in conductive patterns on automotive window glass disclosed in JP 05009623 A, which discloses a composition including a mixture of fine particles containing (a) metallic silver, (b) glass frit having a softening point of 350 to 620° C., and (c) 0.01 to 10 wt %, based on the base paste, of the oxides of transition metals selected from among vanadium, manganese, iron, cobalt and mixtures thereof; wherein components (a), (b) and (c) are dispersed in (d) an organic medium, and the transition metals have a valence of 2+ or higher.
Other art of of interest includes U.S. Pat. No. 5,601,638 to Fukada et al. that teaches a thick film paste that may be used in forming a conductor, resistor, insulator, or protector or the like in a ceramic wiring substrate by a screen printing process. However, the Fukada et al. thick film paste requires the use of a resin and a solvent containing 2-tetradecanol or a mixture of 2-tetradecanol with another solvent.
U.S. Pat. No. 5,616,173 to Okamoto et. al. teaches a thick film composition used to apply a conductive pattern that contains fine particle metallic silver and fine particle glass frit having a softening point of 350–620° C. The glass frit is characterized in Okamoto et al. as an amorphous glass. Its composition is not of importance so long as the transition metal oxide is contained in the glass.
U.S. Pat. No. 4,446,059 to Eustice et. al. teaches a conductor composition comprising an admixture of finely divided conductive metal, an amorphous glass binder, and a colorant which is a mixture of B2O3, copper oxide, and/or a precursor of copper oxide. Again, the composition of the glass binder is not critical to its function.
U.S. Pat. No. 5,296,413 to Carroll et. al teaches a thick film composition for applying conductive patterns comprising finely divided metallic silver particles, glass frit, selected transitional metal oxides, and organic medium. Again the nature of the amorphous glass binders is not critical to the invention.
Typically in this art, the “hot wire” for defogging is formed on the rear window glass and plated to provide the desired weather resistance and resistivity. The “hot wire” is composed primarily of sintered silver and, in the absence of problems relating to resistivity and weatherability, is used without plating. In car models in which the rear windows can be raised and lowered, the hot wire may be subject to wear due to rubbing during raising and lowering of the window. A plating must be applied to the silver conductor to prevent such wear.
In cases where a composition containing a large amount of inorganic material or the like has been employed to enable the hot wire material to withstand wear without having to be plated, the resulting hot wire has often been unable to satisfy other required properties, particularly resistivity, and has thus been unfit for use. However, because of environmental concerns and the need to reduce costs by doing away with plating, there remains a strong desire for a material which has sufficient wear resistance and is capable of satisfying the performance requirements of “hot wires.”
Accordingly, the object of the invention is to provide a material which has sufficient wear resistance even when not plated, and can satisfy the performance requirements for hot wires. “Performance requirements,” as used herein, refers to such characteristics as resistivity, percent change in resistivity, weather resistance, coloration and printability.
The resistance of the conductive pattern of the present invention is required to be on the order of 2 to 30 millions per square. This requirement is readily compatible with precious metal conductors, especially silver, as noted above. Furthermore, the present invention requires a volume resistivity that is 12 μΩ cm or less and a durability (meaning an absence of breaks in the conductor) of at least 100 rubbing cycles. Additionally, the conductive grid wires, which are contained in the window, must be small enough to maintain visibility through the window.